The chemical structures, optical properties and laser-induced damage thresholds of magnesium fluoride films annealed at different temperatures were investigated. The results showed that the stoichiometry of MgF2 film changed a little with the increase in annealing temperature. Analysis of the optical properties indicated that excellent antireflection behavior of the film in the range of 200-400 nm can be obtained by the samples coated with MgF2 film. The refractive index increased and the extinction coefficient decreased with increasing annealing temperature. Compared with the asdeposited films, the laser-induced damage threshold was improved after annealing process and decreased with the increase in annealing temperature, which was probably due to the denser film and more absorption centers under higher annealing temperature.
The solidification behavior of Ti-48Al-8Nb alloy under nonequilibrium solidification conditions was studied by electromagnetic levitation technique.The solidification conditions are different undercooling（△T）under the same cooling condition and different cooling methods at the same undercooling condition,respectively.In different undercooling conditions,when the undercooling is above a critical value（△T＊≈211K）,a remarkable morphological transition from typical hypoperitectic solidification to a sole solidification of the（3 phase resulting in the suppression of the peritectic reaction occurs.For melts with different cooling conditions at the same undercooling（△T≈85 K）,the melt was rapidly cooled by quenching them in cooling media.With cooling rate increasing,a transition from β phase to peritectic α phase solidification mode is revealed for Ti-48Al-8Nb alloy.
Low density ZnO nanorods are grown by modified chemical vapor deposition on silicon substrates using gold as a catalyst.We use high resolution photoiuminescence spectroscopy to gain the optical properties of these nanorods in large scale.The as-grown samples show sharp near-band-gap luminescence with a full width at half maximum of bound exciton peaks at about 300μeV,and the ratio of ultraviolet/yellow luminescence larger than 100.Highly spatial and spectral resolved scanning electron microscope-cathodoluminescence is performed to excite the ZnO nanorods in single rods or different positions of single rods with the vapour-solid grovth mechanism.The bottom of the nanorod has a 3.31-eV luminescence,which indicates that basal plane stacking faults are related to the defects that are created at the first stage of growth due to the misfit between ZnO and Si.
XIE YongJIE Wan-QiWANG TaoWIEDENMANN MichaelNEUSCHL BenjaminMADEL ManfredWANG Ya-BinFENEBERG MartinTHONKE Klaus
The nucleation of eutectic crystals of hypoeutectic A1-7 wt pct Si-Mg casting alloys modified by Ce and Sr was studied by using differential scanning calorimeter （DSC） and scanning electron microscope （SEM）. DSC results were applied to calculate the values of activity energy and nucleation work of eutectic nucleation for the alloys. These values were decreased and the eutectic nucleation frequency was increased with the addition of Ce and Sr to the alloys. Moreover, the morphology of eutectic silicon in the modified alloys was partially or fully modified as fine, fibrous or coral, and DSC plots show that the eutectic undercooling of A1-7 wt pct Si-Mg alloys were increased with additions of modified agents. The mechanism of eutectic modification is combined actions of the accelerated nucleation and the restricted growth.
Quantitative analysis of deformation-induced textures and texture-induced mechanical properties is an important issue for optimal design and control of plastic forming of metals.Deformation-induced textures were predicted through the crystal plasticity finite-element method(CPFEM)in this study,and varying deformation modes,including uniaxial compression,uniaxial tension,simple shear,and plane-strain compression,were considered.The predicted textures were proven by experiments.Then,a theoretical model was proposed to build the quantitative relation between textures and the corresponding mechanical properties.This model takes into account the effects of grain’s orientation,grain’s interaction,and the property in the level of single grain.It captures the macroscopic anisotropy owing to textures and microscopic anisotropy owing to crystallographic structures.By applying this model,the macroscopic stress responses of grains’aggregate with varying textures were calculated according to grain’s orientations and the intrinsic properties of the single crystal along[100]and[111]crystallographic directions.The theoretical model is proven to have high efficiency and acceptable accuracy in the prediction of texture-induced mechanical properties comparing with CPFEM model.
In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the con- stituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determi- nes the phase fraction for a certain initial structure. ^--at-~3 transformation occurs during heating and holding. [3 to ct transformation leads to a significant increase in content and size of lamellar ~. The ct to [3 transformation occurs simultaneously in equiaxed at and lamellar ct. The thickness of lamellar ~t increases with temperature, which is caused by the vanishing of fine a lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.
Xiao-Guang FanHuo-Jun ZhengPeng-Fei GaoMei ZhanWen-Jia Mei
